The dehydrogenation of hydrocarbons is an important reaction. For example, most of lower olefins are obtained through the dehydrogenation of lower alkanes. The dehydrogenation can be classified into two types, direct dehydrogenation (not involving oxygen) or oxidation dehydrogenation (involving oxygen).
It has been proved that several types of nano-carbon materials have catalytic effects on the direct dehydrogenation and the oxidation dehydrogenation of hydrocarbons. Incorporation of oxygen and/or nitrogen atoms into the nano-carbon material can improve its catalytic effect.
Incorporation of oxygen atoms into the nano-carbon material can form oxygen-containing functional groups such as hydroxyl, carbonyl, carboxyl, ester and anhydride groups on the surface of the nano-carbon material.
The nano-carbon material can be oxidized to introduce the oxygen atom into the nano-carbon material and increase the amount of the oxygen-containing functional groups in the nano-carbon material. For example, the nano-carbon material can be treated in a reflux condition of a strong acid (e.g. HNO3, H2SO4) and/or a strong oxidative solution (e.g. H2O2, KMnO4) optionally in the help of the microwave heating or the ultrasonic oscillation to enhance the oxidation effect. However, the reflux treatment in the strong acid and/or the strong oxidative solution will have a negative effect on the framework structure of the nano-carbon material, and even destroy the framework structure of the nano-carbon material. For example, the reflux treatment of the nano-carbon material in a nitric acid solution can introduce a large amount of oxygen-containing functional groups to the surface of the nano-carbon material, but said treatment will be apt to cut off the nano-carbon material and/or remarkably increase the defect sites in the graphite network structure, and therefore reduce the properties of the nano-carbon material, e.g. thermostability. In addition, the amount of the oxygen atoms introduced by the reflux treatment in the strong acid and/or the strong oxidative solution has a high dependency on the reaction conditions, and will fluctuate widely.
For the introduction of the nitrogen atom into the nano-carbon material, according to the chemical environment in which the nitrogen atoms in the nano-carbon material exist, the nitrogen atoms can be divided into the chemical nitrogens and the structural nitrogens. The chemical nitrogens are mainly present on the material surface in form of the surface functional groups, e.g. the surface nitrogen-containing functional groups such as amino or nitrosyl. The structural nitrogen means the nitrogen atom is present in the framework structure of the nano-carbon material and bonded to the carbon atom(s). The structural nitrogen mainly comprises the graphite-type nitrogen
the pyridine-type nitrogen
and the pyrrole-type nitrogen
The graphite-type nitrogen directly replaces the carbon atom in the lattice of the graphite to form a saturated nitrogen atom; the pyridine-type nitrogen and the pyrrole-type nitrogen are unsaturated nitrogen atoms, and will usually cause the deficiency of the adjacent carbon atoms upon replacing the carbon atom to form defect sites.
The nitrogen element can be introduced to the framework structure and/or the surface of the nano-carbon material with the high temperature and/or the high pressure in the synthesis process of the nano-carbon material by introducing a nitrogen-containing functional atmosphere (e.g. ammonia, nitrogen, urea, and melamine) in the synthesis process of the nano-carbon material; or the nitrogen element can be introduced to the surface of the nano-carbon material with the high temperature and/or the high pressure by placing the nano-carbon material in a nitrogen-containing functional atmosphere (e.g. ammonia, nitrogen, urea, and melamine). The high temperature and/or the high pressure can form the structural nitrogen of the nano-carbon material; however the type of the nitrogen-containing species depends on the reaction conditions and is not easily controllable. Moreover, different types of the formed nitrogen-containing species are not evenly distributed on the surface of the nano-carbon material, leading to the instability of the properties of the nitrogen-containing nano-carbon material. The nano-carbon material can be also oxidized and then reacted with an amine to introduce the nitrogen atom to the surface of the nano-carbon material. The introduced nitrogen atom is substantially the chemical nitrogen.
Although there are some processes in the investigations on the doping-modified nano-carbon material and the catalytic capability thereof, however the scientists and reservations have not reached the consensus for some basic core issues, and there is still a need to further investigate the doping-modified nano-carbon material, the preparation process thereof and the catalytic capability thereof.